Its not a joke, its just a message, telling people that being happy doesnt have to rely on wealth and fancy stuff. That there is nothing wrong with being happy with a simple life.
I also expected a joke from a comic. But I read the whole thing, and when there wasn't a joke, I didn't mind. T'was a beautiful journey. Definitely wasn't for nothing, for me.
I think the punchline is that she can be distracted from an existential moment by something as simple as chicken nuggets because they are exactly the kind of simple pleasure thst makes her life with her partner worth living. I think it's rather sweet.